Continuous or spot measurements of body core temperature through tympanic membrane (eardrum) thermometry generally require unrestricted access to the ear canal so that the temperature sensor's field of view can be completely filled by the tympanic membrane. In cases where the ear canal is obstructed by other components, or the subject presents different physiology, it can be very challenging for the temperature sensor itself (e.g., a thermopile) to reach all the way to the eardrum. As the standoff distance from the eardrum increases, the percentage of the region of interest diminishes as the square of the standoff distance, thereby rapidly increasing the difficulty in obtaining a reliable reading with a high degree of confidence. There is therefore a need for an optical design that allows the sensor to be located less deeply into the ear canal or even outside the ear canal, and yet still allows for the sensor's field of view to be substantially filled by the tympanic membrane.